The present invention relates to a method for manufacturing a thin film transistor, and more particularly to a method structure for crystallizing a film using a lens which is applicable to a SRAM (static random access memory) and liquid crystal display devices.
In the conventional method for manufacturing a thin film transistor, a bottom gate and a top gate are used. A polysilicon for forming a body layer is manufactured by enlarging the particle size using solid phase growing method. The solid phase growing method for the body polysilicon layer according to the conventional method is carried out by heat treating at about 600xc2x0 C for about 24 hours.
Recently, an attempt to increase the crystallization degree by RTA (rapid thermal annealing) is made. However, this increases production cost and is difficult to proceed in batch type.
A plasma H2 passivation used to be applied to obtain on/off current ratio (Ion/Ioff) of 106 or above.
The conventional method for manufacturing a thin film transistor will be described below referring to attached FIGS. 1a-1c. 
First, a gate electrode 2 is formed by depositing a conductive material such as polysilicon on an insulating layer 1 formed on a substrate (not shown) and patterning; through photolithography as shown in FIG. 1a. 
After then, a gate oxide 3 and a body polysilicon layer 4 are sequentially formed by CVD (chemical vapor deposition) on the whole surface of the insulating layer 1 in which the gate electrode 2 is formed, and channel ions are implanted.
As shown in FIG. 1b, photoresist 5 is coated on the body polysilicon layer 4 and is selectively exposed and developed to form a prescribed photoresist pattern to define a channel region and an offset region on the body polysilicon layer 4. At this time, the region covered with the photoresist 5 becomes the channel region and the offset region. Ion implantation is carried out to form an LDD (lightly doped drain) region 11 using the photoresist pattern 5 defining the channel region and the offset region as an ion implantation mask.
As shown in FIG. 1c, the photoresist pattern 5 defining the channel region and the offset region is removed and then photoresist 6 is coated again, selectively exposed and developed to form a photoresist pattern 6 defining a source region and a drain region. Ion implantation is carried out using this photoresist pattern 6 as a mask to form the source region and the drain region 12 to complete the manufacture of the thin film transistor as shown in FIG. 1c. 
FIG. 2 is a schematic cross-sectional view of the channel region formed on the body polysilicon layer 4 of the thin film transistor manufactured by the conventional method.
According to the conventional method, a problem of leakage current generation by the grain boundary A which passes through the source S and the drain D as shown in FIG. 2 occurs.
Moreover, since the offset region and the source and the drain region are defined using the photoresist, the length of the channel region and the length of the offset region are changed according to the degree of overlay misalignment, and stable device characteristic could be hardly obtained.
An object of the present invention is to solve the above-mentioned and other problems encountered in the conventional methods and to provide a method for manufacturing a thin film transistor having high reliance and high reproductivity.
Another object of the present invention is to provide a method and structure for crystallizing a film using a lens.
To accomplish the above and other objects, there is provided in the present invention a method for manufacturing a thin film transistor comprising the steps of forming a polysilicon layer on a substrate, forming an impurity-containing layer on the polysilicon layer, forming a prescribed pattern by patterning the impurity-containing layer, flowing the impurity-containing layer by heat treating and crystallizing the polysilicone layer by annealing using a prescribed light source.
Briefly described, the present invention is directed to a method for crystallizing a film, including steps of forming a film on a substrate; forming a lens on the film to focus an electro-magnetic wave onto the film; and directing the electromagnetic wave on the film inclusive of the lens to crystallize the film.
Furthermore, the present invention is directed to a structure for crystallizing a film, including: a film formed on a substrate, and a lens formed on a predetermined portion of the film to focus an electro-magnetic wave on the predetermined portion of the film.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.